Question for paddlers who use a GPS. What is your experience in guidebook miles vs real miles? Or in predictive miles; is that type of planning is possible with a GPS?
Accurate measurements via Google Earth do it for me.
I don't use a GPS for recreational paddling, as in general I find those things distracting and unnecessary to more traditional and enjoyable observational navigation. But I do use a GPS for marathon canoe racing. Mostly to be sure I maintain my speed goals, and in the bigger races also to know how far in distance/time to the next major loaded waypoint.
I've paddled the big Yukon RIver races several times. Using a GPS there is critical to competing. Existing topo maps are horribly inaccurate due to the age of the existing maps and the vastly changing character of the river. The long published distance from Whitehorse to Dawson City is 460 miles, based on the old paddle wheelers best estimate in the main river channel. GPS mileage as measured by several GPS units over 4 years on the Yukon races consistently comes out to 426 miles to Dawson. The significant difference is partially understandable and expected since paddlers will naturally take shortcuts out of the main channel, but I'm not sure you can account for 34 miles of difference that way.
Google Earth, on the other hand, shows accurately registered images of the river that are only a couple of years old in most places (as opposed to the decades old topo maps). When paddling/racing on the Yukon it is extremely important to know where you are, to set up for which side of the upcoming gravel bar to be on, or which side of the river for an upcoming turn and best current. The river is so wide and current so strong, you can't simply wander from shore to shore without a detailed route plan.
I use the Google Earth ruler/path function to set waypoints and accurately measure the exact track I want to take. GE map measured distance to Dawson is 419 miles, vs actual GPS measurement of 426 miles. Not bad, a difference factor of 1.016. I attribute the slight difference to wandering in our paddle course and carving turns. Going even further to the finish of the Yukon 1000 mile race, that same factor holds true to compare actual GPS with the ever slightly shorter GE map measurement.
So I use this to my advantage. I carefully plot the route on GE and have GE create a database of the waypoints coordinates so made, in this case 446 waypoints to get me around river bends and islands from Whitehorse to Dawson (738 points to Dalton Bridge). The spreadsheet calculates distance and names waypoints by their distance in tenths of a mile from Whitehorse, and also includes in the waypoint name the upcoming turn direction, and desired river left/right information. This route and waypoint list is loaded into the GPS. I also print and waterproof a map from GE with the same route and labeled waypoints. This makes it very easy to glance at the GPS display with the upcoming waypoint showing direction and distance to next, and the name correlates to the printed map waypoint if I should need to refer to a larger picture. The GPS odometer also corresponds to the waypoint name upon arrival. In practice one GPS is displaying direction and course, another is displaying the map.
The spreadsheet also predicts and adjusts, based on our actual location and SPOT transmissions, the expected arrival times at various upcoming points of interest (including the finish) for my pit crew. It is quite accurate and very cool.
